"We have managed to agree on the main issues," Russian President Vladimir Putin said.

The four leaders had committed to respect Ukraine's sovereignty and territorial integrity, according to a joint declaration distributed by the Kremlin.

"The main thing which has been achieved is that from Saturday into Sunday there should be declared without any conditions at all, a general ceasefire," Ukrainian President Petro Poroshenko told journalists.

French President Francois Hollande said there was still much work to be done on the Ukraine crisis, but the agreement was a real chance to ameliorate the situation.

He said pro-Russian separatists, who had at one point appeared to reject the deal, had signed up to it.

Merkel and Hollande joined Poroshenko and Putin for a marathon negotiating session that began early on Wednesday evening and continued into Thursday morning.

The summit discussions came as pro-Moscow separatists tightened the pressure on Kiev by launching some of the war's worst fighting on Wednesday, killing 19 Ukrainian soldiers in assaults near the railway town of Debaltseve.

As the fighting escalated, Washington has begun openly talking of arming Ukraine to defend itself from "Russian aggression", raising the prospect of a proxy war in the heart of Europe between Cold War foes.

The outcome of the Minsk talks is expected to influence discussions at an EU summit in Brussels on Thursday, when sanctions against Moscow will be on the agenda. A deal would likely mean a softer line toward Moscow.

Merkel and Hollande were expected to arrive late for the Brussels meeting but would urge the European Union to support the Minsk deal.